In this paper, we introduce the idea of using unmanned aerial vehicle (UAV)-based free-space optical communication systems to backhaul high-speed trains. We introduce a composite channel model that includes effects of both atmospheric turbulence and pointing errors due to position/orientation deviation. Based on the derived fading model, we present an approximation expression for the outage probability. The performances of the proposed air-relay and traditional ground-relay systems are compared under fair conditions. Pointing-acquisition-tracking (PAT) complexity and coverage distance under different weather conditions and different fading levels are considered as comparison metrics. Furthermore, we investigate the effect of several parameters such as beam divergence angle, displacement deviation variance, and UAV operation altitude on system performance. Our results reveal that air relay helps in mitigating the fog effect, can achieve longer coverage distance, and relaxes PAT system design.